A. Field of the Invention
The present invention is directed to a vertical light emitting diode (LED) device, and more particularly to a high-brightness LED device having an outwardly located metal electrode.
B. Description of the Prior Art
Currently, LED is widely applied in our daily life due to its characteristics of low production cost, easy fabrication, small size, low power consumption and high efficiency, for example, in the fields of cell phones, electric boards, electric torches, traffic lights and so on. Nevertheless, improvements in the luminous efficiency and brightness of a LED are pursued continuously.
Recently, high-brightness LEDs using nitrides and phosphides have been developed, which can not only emit red, green and blue light but also produce light in various colors and white light. At present, LED lighting applications are developed enthusiastically by industry. In the early stage of manufacturing development, multiple LEDs were combined to form an array, so as to achieve high output power. However, in terms of manufacturing, a LED device including a LED array is more complicated than a single LED device with high output power. Therefore, the cost of manufacturing the LED array device is higher and the stable reliability is less likely to be achieved.
One method to increase the power and luminous flux of a LED is to increase the size and luminous surface area thereof. However, the semiconductor material layer used in a conventional LED usually has poor conductivity, such that electric current cannot be spread effectively and uniformly over an active layer from a contact. Therefore, some areas inside the LED can produce high electric current density phenomenon, thereby affecting the whole brightness, even leading to early deterioration in the proximity of the active layer. As a result, the service life of the LED is reduced significantly.
FIG. 1A is a top view of a configuration of a conventional small-size vertical LED device 100. FIG. 1B is a cross-sectional view of the configuration of the LED device 100 shown in FIG. 1A. FIG. 2 is a top view of a configuration of a conventional large-size vertical LED device 200. With reference to FIG. 1B, the configuration of the conventional small-size LED device 100 typically includes a first electrode 109, a conductive base layer 108 formed on the first electrode 109, a reflective minor layer 106 formed on the conductive base layer 108, a first conductivity type semiconductor layer 104 formed on the reflective mirror layer 106, an active layer 103 (or referred as an emission layer) formed on the first conductivity type semiconductor layer 104, a second conductivity type semiconductor layer 102 formed on the active layer 103, and a second metal electrode 101 formed on the second conductivity type semiconductor layer 102. As shown in FIG. 1A, in the small-size vertical LED device 100, the second metal electrode 101 is located on the center of the second conductivity type semiconductor layer 102. Furthermore, additional metal wires are not required due to the small size and the good current spreading performance of the LED device 100.
For a conventional large-size vertical LED device, a major reason of affecting the luminous efficiency of the LED device is the failure to spread electric current uniformly, so it is contemplated to increase the thickness of a semiconductor material layer so as to increase the conductivity. For the small-size LED (less than about 0.25 mm2) shown in FIGS. 1A and 1B, its brightness and current spreading performance can certainly be improved by this method. However, the increased thickness of the semiconductor material layer may not only increase production costs but also lead to stress problems. Therefore, it is impossible to unlimitedly increase the thickness of the semiconductor material layer to comply with the current spreading performance requirement of a large-size LED device. As a result, for the large-size device shown in FIG. 2, a satisfactory performance cannot be achieved merely by increasing the thickness of the semiconductor material layer. This is because when the size of a LED device is increased, it becomes more unlikely to uniformly spread electric current over the semiconductor material layer from an n-type contact or a p-type contact. It can be seen that the size of a LED is substantially limited by the current spreading characteristic of the semiconductor material layer.
As shown in FIG. 2, in a conventional large-size vertical LED device 200, a second metal electrode pad area 210 is located on the center of a second conductivity type semiconductor layer 202, which generally utilizes radial metal electrodes 201 to increase the current spreading performance. However, most of the outlines of common LED devices are squares or rectangles, therefore it is difficult not only to place each radial metal wire on an emission layer such that the best current spreading performance can be achieved, but also to ensure that the adjacent radial metal wires have constant interval therebetween. In addition, both sides of the metal electrode are high illumination sides, which tend to absorb emission light, thereby decreasing brightness. As shown in FIGS. 3A and 3B, for other conventional large-size vertical LED devices 200A and 200B, both sides of their metal electrodes are high illumination sides, which also tend to absorb emission light, thereby decreasing brightness. Therefore, conventional LED devices still generally have the following problems including uneven current density, low light extraction efficiency, unsatisfactory brightness, unsatisfactory efficiency, short service life, and so on, which are to be solved.